In my prior U.S. Pat. No. 3,774,215, assigned to the same assignee as the present application and specifically incorporated herein by reference, there is disclosed a system for providing position information between a plurality of mobile vehicles and a central station using Omega transmissions and time-shared data communication between the mobile vehicles and a central station. Applications for data communication between a central or base station and a plurality of mobile satellite stations may occur where exact position information as obtained by the Omega system is not required, but instead a full information exchange capability in both data and voice formats is desired. Such applications may be found in the trucking industry or in regional transit systems.
The typical trucking system can greatly benefit from having available at the dispatcher's office a variety of information on the condition of the individual trucks including the total load-on-board, bill of lading and routing sequence of the truck, status or emergency conditions of vehicle engine or other components. Additionally, the dispatcher may wish to transmit data to the vehicle to specify certain actions to be performed, such as route changes or to send communications coordination signals for voice operation, such as ring and channel switching commands. A large repertory of predetermined data messages is thus needed for communication between the vehicles. By using such a repertory in a time-shared data format, it should be possible to communicate with a larger number of mobile vehicles using time-sharing or multiplexing techniques.
Conditions can be expected, however, which cannot be satisfactorily handled by communicating through a predetermined repertory of data messages and which necessitate the occasional use of direct voice communication. In such cases, it is desirable to have a system which provides for occasional interruption of the more efficient time-shared data communication on a priority basis to permit direct voice communications between a dispatcher and a particular mobile vehicle. Since dual frequency assignments are less available than single frequency assignments, the use of a single frequency for both voice and data is desired.